Loess Hills

The Loess Hills are a formation of wind-deposited loess soil in the westernmost part of Iowa and Missouri along the Missouri River.

Geology

The Loess (/ˈloʊ.əs/, /ˈlʌs/, or /ˈlɛs/) Hills are generally located between 1 and 15 miles (2 and 24 km) east of the Missouri River channel. The Loess Hills rise 200 feet (60 m) above the flat plains forming a narrow band running north-south 200 miles (320 km) along the Missouri River. These hills are the first rise in land beyond the flood plain, forming something of a "front range" for Iowa, and parts of Missouri and Nebraska adjacent to the Missouri River.

During the last Ice Age, glaciers advanced into the middle of North America, grinding underlying rock into dust-like "glacial flour." As temperatures warmed, the glaciers retreated and vast amounts of meltwater and sediment flooded the Missouri River Valley. The sediment was deposited on the flood plain, creating huge mud flats. When meltwaters receded, these mud flats were exposed. As they dried, the fine-grained silt was picked up by strong prevailing westerly winds. Huge dust clouds were moved and redeposited over broad areas. The heavier, coarser silt was deposited close to the Missouri River flood plain, forming vast dune fields. The dune fields were eventually stabilized by grass. Due to the erosive nature of loess soil and its ability to stand in vertical columns when dry, the stabilized dunes were eroded into the corrugated, sharply dissected bluffs we see today.